1. Field of the Invention
The present invention relates to a magnetostrictive transducer for generating torsional waves in a rod member using magnetostriction, and receiving a signal transmitted by the rod member due to an inverse magnetostrictive effect.
2. Description of the Related Art
Magnetostriction or the Joule effect is an effect in which a physical change occurs in ferromagnetic materials when they are subjected to a magnetic field. The reciprocal effect, the change of the internal magnetic state of ferromagnetic materials when they are subjected to a mechanical stress, is called the Villari effect.
A transducer using magnetostrictive effect can measure changes in an object without making physical contact with the object, and is thus used in diverse fields where contact sensors cannot be used. Ultrasonic waves that can be generated in a waveguide, such as a rod or a pipe, include longitudinal waves, lateral waves, and torsional waves.
Referring to FIG. 1, a conventional magnetostrictive torsional wave transducer includes a thin strip 1 wound around a rod member 2 in a direction perpendicular to the axis of the rod member 2, an insulator 4 wrapped around the strip 1, and a coil 3 wound around the insulator 4. The strip 1 is made of a ferromagnetic material such as nickel, iron, cobalt, or an alloy of them.
When torsional waves are generated using the conventional magnetostrictive transducer to diagnose a defect in the rod member 2, the strip 1 is fixed around a cylindrical pipe and then the strip 1 is rubbed in the circumferential direction using a permanent magnet or the like to magnetize the ferromagnetic strip 1. A magnetic field is applied around the magnetized strip 1 via the coil 3, thereby generating an elastic wave. When the magnetic field is applied to the strip 1 , a torsional wave is induced in the rod member 2. The torsional wave propagates through the rod member 2, and is reflected at an end or a structural defect of the rod member 2. The magnetic field around the strip 1 changes due to the reflected torsional wave, and this change can be measured.
As described above, in the conventional magnetostrictive transducer, a bias magnetic field is generated when the strip 1 is rubbed with a permanent magnet in a circumferential direction. However, when pre-magnetization is achieved by rubbing the strip 1, the degree of magnetization is not the same at every point of the strip, and it is difficult to form a uniform magnetic field throughout the strip 1. In addition, when a large amount of current is applied, the intensity and direction of the initialmagnetic field changes, and therefore, re-magnetization is required. In particular, a rod member, such as an oil pipeline, is buried underground, it is not easy to approach, and it is difficult to repeatedly re-magnetize the strip 1. Most of all, the quantity of the magnetization of the strip can be increased by the direct supply of external magnetic field.
Accordingly, a method that efficently magnetizes the strip 1 without having to rub is required. In other words, it is highly desirable to develop a method of generating/measuring a torsional wave more effectively by providing a bias magnetic field in the strip 1 in a stable, continuous manner.